The present invention relates to a device to be mounted in sewing units and intended for adjusting plies of material, which are to be sewn together by means of a sewing machine and which register with each other at their leading edges, to mutually equal lengths and/or for holding them in such adjusted equal length position.
While sewing together plies of material, for example, cut pieces such as used in the manufacture of trousers, to obtain a faultless working result, it is necessary to sew the plies together so as to bring not only their outlines but also their lengths into registration. Differences in length may occur so that, in spite of an exactly uniform operation of the feed tools, the plies of material are fed at unequal advance velocities due to unequal friction conditions, with the result of a mutual displacement of the plies during the sewing operation. However, it may happen that already the initial lengths of the plies are different, because of an inaccurate cutting.
In a known sewing unit, prior to the sewing operation, the leading edges of the plies of material are clamped in registry with each other between the feed tools of the sewing machine, and the trailing edges are clamped, also in registry with each other, into a horizontally movable, weight loaded, trailing clamp. Due to the stretching of the two plies caused by the trailing clamp, a small displacement of the plies can be compensated while, during the sewing operation, the ply advanced at a lower speed contracts again, without forming wrinkles. Further, by means of the trailing clamp, if the pieces of material are cut inaccurately, to unequal lengths, the plies can be adjusted in length before starting the sewing operation or their lengths can be maintained during the sewing operation. The trailing edges may, in this case, be brought into registration, or mutually adjusted, so that either the shorter ply is manually stretched to the measure of the longer one or the longer ply is adjusted by upsetting or creasing to the shorter ply with, in the latter case, the stretching of the plies being produced by the trailing clamp. While, for stretching the shorter ply of material, a pull must be exerted thereon by the operator, in the second case, the operator has to exert a pull on the trailing clamp in order to displace it from the initial position thereof to the trailing edges of the two plies. Thus, the alignment and clamping of the plies in the trailing clamp requires, in any case, a more or less considerable exertion of force on the part of the operator and a certain amount of attention so that, particularly if pieces are involved having unequal lengths already prior to the sewing operation, the quality of the resulting work still depends on the reliability and skill of the operator, in spite of a partial mechanization of the operations.